A fastening screw is used in a variety of industrial fields since it can be used for connecting a plurality of members with ease. Particularly, a tapping screw immediately fastens steel plates or synthetic resin material only with forming a pre-drilled hole and without forming an internal thread to the material. That is, no tapping operation or tool like a screw tap is required to form the thread. Therefore, the tapping screw is widely used in manufacturing household electric appliances, automobiles and the like.
Meanwhile, in this kind of screw, generally, torque for loosening the screw is small and the screw is susceptible to inverse rotation or loosening due to vibration after tightened.
To solve the problem, a spring washer, double nuts or the like are applied when bolts and nuts are used. Further, when the tapping screw is adapted, a locking washer or bonding agent is preferably adapted.
However, with the conventional countermeasures described above, separate parts from the screw such as spring washers, another nut, other washers, and bonding agents are required, which increases costs and manhour for fastening work. Moreover, when the tapping screw is used, plastic deformation of an internal thread material loosens the screw even though the screw does not rotate. Therefore, with only the above method, the loosening can not be prevented securely, which provides another problem in this respect.
The present invention has been accomplished to eliminate the above drawbacks and the object thereof is to provide a screw, and a thread rolling die for manufacturing the screw.